The present invention relates to laser beam irradiation equipment, and in particular, it relates to submerged laser beam irradiation equipment capable of expelling water from a space between a submerged laser beam irradiation nozzle and the surface of a submerged workpiece and preventing water intrusion into said space.
Although their heat energy sources may differ, many types of submerged laser beam processing equipment, which process a submerged workpiece by irradiating it with a laser beam to heat and melt the surface of the workpiece, can employ the same localized water repulsion technique which is used in submerged arc welding. According to the conventional localized water repulsion technique, since the presence of any gap between the water repulsion nozzle and the surface of the workpiece will permit water intrusion, a constant mechanical load must be applied to the water repulsion nozzle to maintain it in contact with the surface of the workpiece under pressure so that an air chamber is maintained locally to enable processing under the submerged environment.
Further, equipment is presently in use wherein, by ejecting a fluid or gas along the circumference of the water repulsion nozzle, an air chamber is created which is isolated locally from water intrusion even if there exists a gap between the water repulsion nozzle and the surface of the workpiece, thereby enabling submerged processing.
JP-A-No. 49-98746 discloses a water tight nozzle having a curtain wall including a plurality of curtain members attached around an external circumference of a guide tube for guiding its welding core wire, wherein the plurality of curtain members are arranged densely in contact with each other in a liquid-tight manner and slidably in vertical directions so that each member moves independently in the vertical direction urged by its own weight or under pressure applied externally.
This prior method, however, does not take into account a problem that, since the plurality of curtain members are arranged perpendicular to the surface of the workpiece, there occurs rubbing between the plurality of curtain members and the surface of the workpiece when the torch is moved, thereby bending some of the plurality of curtain members. Further, it does not take into account the problem which occurs when a gap is formed between the curtain members and the surface of the workpiece due to such bending, which allows water intrusion, and the problem that inward bending of the curtain members to the side to which the torch is advancing may cause a short circuit with the arc. Further, according to this prior equipment, during the time the nozzle is maintained in a face down attitude, it is urged into close contact with the surface of the workpiece by its own weight, however, when the nozzle attitude is slanted, a mechanism to press it into contact with the workpiece surface is required.
JP-A-No. 49-023133 discloses a welding torch having a blade runner provided in the outer circumference of a shield gas nozzle, which blade runner is driven at a high speed by a mechanical drive source in order to expel water from a space immediately below the welding torch and the surface of the workpiece to form a vapor phase region to enable submerged welding therein.
According to this prior method, water expulsion is achieved by applying a rotating force to water present around the outer circumference of the nozzle and water present in a gap between the blade runner and the surface of the workpiece. Therefore, it becomes necessary to maintain an appropriate relationship between the gap and the rotating force to balance both lest there should occur water intrusion, as well as to control the gap to be always within a permissible range in order to prevent damage to the surface of the workpiece due to any contact by the surface with the blade runner. Further, occurrence of water intrusion can be expected due to other causes, such as the presence of irregularities on the surface of the workpiece which permit water intrusion through gaps caused by these irregularities immediately below the nozzle, or due to changes in the nozzle attitude to cause it to face upward or sideward, which may also form a gap due to gravity between the upper portion of the blade runner and the surface of the workpiece.
JP-A-No. 7-100673, which relates to submerged laser beam irradiation equipment, discloses a nozzle head having around its annular periphery an annular gas ejector having gas ejection ports for ejecting gas annularly. Gap control between the nozzle head and the surface of a workpiece, which is also required in this prior method, is accomplished by provision of a magnetic wheel which travels by rotating as magnetized. In this case, the workpiece is limited to a magnetic material.
Further, no prior art is known which discloses a method for preventing water intrusion into the nozzle head while the head is moved from the atmosphere to a specific processing position in the water. The prior nozzle has such a structure that, even if water is expelled during submerged welding, when the ejection of the water expulsion fluid is stopped, water intrusion into the welding head cannot be avoided. This disadvantage, which is associated with conventional arc welding, is not limited thereto, but also happens in the case of laser beam welding.
For conventional submerged processing using the above-mentioned submerged laser beam irradiation nozzle, it is required to be able easily to expel water from the nozzle chamber irrespective of the surface condition of the workpiece, such as the presence of irregularities, and of the nozzle attitude during processing. In addition, since the laser irradiation nozzle moves at an arbitrary speed during processing, and thereby a processing area from which water must be expelled will move accordingly, the surface profile, such as irregularities on the workpiece to be irradiated, has a large influence on the effect of water expulsion. Accordingly, there is a need to develop an efficient water expulsion and water immersion prevention method.
In particular, in the case of laser beam irradiation for use in repairing a welded structure or for surface reforming a heat affected zone, the surface of the welded structure typically is not flat, but has irregularities due to the presence of a stepwise portion produced during staggered butt welding, and an angular deformation caused during welding, the removal of excess weld metal and the like. Thereby, provision of an appropriate water expulsion and water immersion prevention method capable of following such irregularities in the surface is required.
A nozzle press-loading method which presses the nozzle against the surface of a workpiece requires a press-loading mechanism, control of the pressing force in compliance with the presence of irregularities, and use of a flexible press contact member at the nozzle end which contacts the surface of the workpiece. In addition, it is also required to strike a balance between the nozzle travel speed and the flexibility of the press contact member. When the pressing force is too large, the friction resistance produced by a convex portion of the irregularities becomes greater, thereby causing the press contact member to bend. When the press contact member is too rigid, there occurs a problem in that the press contact member under pressure does not make good contact with a concave portion of the irregularities.
In the design of a structure capable of preventing water intrusion into the nozzle chamber, it is important, firstly, to prevent such water intrusion into the nozzle chamber while the laser beam irradiation nozzle is being moved the from atmosphere into the water so as to be positioned opposite to a submerged workpiece, and secondly, it is important to water-tightly press the press contact member into contact with the surface of the submerged workpiece having irregularities, while maintaining a constant distance between the nozzle end and an irradiation surface of the workpiece.
It is also necessary to take measures to ensure that a normal laser beam irradiation will be maintained even when prevention of water intrusion into the nozzle chamber fails, for example, by removing a water film or water droplets formed on a lens protection glass provided at the end of the laser beam irradiation unit by injection of a shield gas into the nozzle chamber. Further, it is preferable to use an inert gas as a water expulsion injection fluid in place of air in order to prevent degradation of the water quality.
However, since it is not economical to keep injecting the inert gas from the time the nozzle is submerged, steps must be also taken to minimize the amount of the inert gas to be injected.
Further, in the case of a workpiece which has irregularities on its surface, it is necessary to precisely control the focal point of the laser beam relative to its surface profile while the nozzle head travels. In particular, it is important to hold the nozzle at a constant focal distance when carrying out welding or surface reforming using a small input of heat in order to assure the quality of the irradiated portions. Further, since injection of a large amount of water expulsion fluid in a closed chamber, such as a tower, vessel and the like, will change the water quality, minimization of such fluid injection must also be considered as described above.
Still further, in the case where a high power laser beam is used for a long duration, and where its mirror tube is comprised of an optically opaque metal, such as copper and aluminum alloy, heat due to the laser beam will be absorbed by the mirror tube material and will accumulate therein even though submerged in water, thereby gradually increasing its temperature. Generally, a laser beam optical system comprises precision components, such as a plurality of lenses and a mirror tube for housing the lenses. The sizes of these precision components will change in response to a temperature change. Since even a small change in size will cause the focal point of the laser beam to alter greatly, the result will be an unstable weld quality due to changes in the depth of fusion or the width of the welding. Further, in the case of laser beam cutting, the result will be a change in the cutting width and an irregular cutting surface.